This invention relates to a gas flow rate measuring apparatus for measuring flow rate (the amount of flow) of gas, and more particularly to a gas flow rate measuring apparatus using a heat-generating temperature sensing resistor to measure air flow rate sucked into the engine of an automobile, etc.
As the most general method for accurately controlling a gas fuel ratio for an internal combustion engine, there is a method for measuring intake gas flow rate and computing one-by-one the injection quantity which makes burning condition optima. A gas flow rate measuring apparatus for measuring intake gas flow rate is indispensable in this.
Thus, up to now, gas flow rate measuring apparatuses using various systems have been proposed, and are widely put to practical use. As one system of them, there is a heat-type gas meter which can detect the mass flow of gas.
In this heat-type gas meter, a temperature sensing resistor located within an intake gas flow path is included as a gas flow rate detecting element, which is electrically connected to a detecting circuit.
This detecting circuit is also called as a control circuit, which supplies a current to the temperature sensing resistor per se or a heater positioned adjacent thereto to heat-generate it, and at that time the amount of the supplied current is controlled so that the temperature of the temperature sensing resistor is always high by a fixed temperature with respect to the temperature of gas sucked into the internal combustion engine.
When the gas flows along the temperature sensing resistor, the amount of heat of which is deprived from the temperature sensing resistor increases, its proportion varies depending upon the rate at which gas flows, and based on this the value of the current supplied to the temperature sensing resistor or the heater also varies.
As a result, the amount of variation of the heating current supplied to the temperature sensing resistor becomes the value corresponding to gas flow rate.
Also, in order to obtain the correct amount of heat radiation from the heat-generating temperature sensing resistor, it is needed to detect the temperature of the gas to be measured, so another temperature sensing resistor is located within the intake gas flow path as an intake gas temperature detecting element.
An electric signal representing the amount of gas flow thus detected is inputted to an output processing circuit to be processed therein so that it conforms with the input voltage range necessary for an engine control device, and it is outputted as the output signal of the gas flow rate measuring apparatus.
An arrangement using such technique is, for example a gas flow rate measuring apparatus described in Japanese Patent Prepublication No. 4487/1997. This gas flow measuring apparatus comprises a module housing including therewithin an electronic circuit board, said module housing being inserted in the inside of an intake pipe, and therefore it constitutes a superior technique in which the heat generated by the electronic circuit board can be cooled by suction gas.
Incidentally, other examples of the gas flow rate measuring apparatus are shown in Japanese Patent Prepublication No. 231899/1993 and Japanese Patent Prepublication No. 503310/1997 (which corresponds to U.S. Pat. No. 5,631,416)
However, although the prior technique shown in above-mentioned Japanese Patent Prepublication No. 4487/1997 has the structure in which the heat generated by the electronic circuit board is cooled by intake gas, consideration for thermal effects which electronic components receive in case where the amount of heat to be cooled varies due to the amount of intake gas is not sufficient.
That is, the thermal type gas flow rate measuring apparatus has a power transistor mounted to the electronic circuit to provide supply current necessary for heat-generation, since it carries out the heat-generation by flowing a current through the temperature sensing resistor.
Because the power transistor produces self heat-generation at the time of current amplification, peripheral members and electronic circuit components receives heat effects thereby.
The electronic circuit board of the gas flow rate measuring apparatus is intended to reduce the number of parts, lower the cost due to the process reduction of mounting works and further make the product smaller by mounting on the same circuit board all electronic circuit parts other than a temperature sensing resistor for detecting gas flow rate and gas temperature.
Therefore, the power transistor is mounted on the electronic circuit board and the heat of this power transistor is thermally conducted to the circuit board on which it is mounted and then to a metallic base for the connection to the circuit board, whereby heat effect is given to the peripheral members and other electronic circuit elements.
Since the gas flow rate measuring apparatus is arranged so that it is positioned mainly within the inside of the intake pipe of an automobile engine, the heat of the electronic circuit board is carried away by intake gas, but the gas flow speed within the intake pipe is different depending upon the magnitude of the amount of intake gas of the engine. Particularly, when the engine is in idling condition, the gas flow speed is relatively slow, so there is produced the problem that in case where the heat of the circuit board is perfectly withdrawn therefrom, the nonconformity of an electronic circuit due to the heating of the circuit board, a measuring error due to temperature dependency each element and resistor, etc. have, or a detection error resulting from the fact that the temperature sensing resistor receives the heat effects, occurs.
An object of this invention is to effectuate a gas flow rate measuring apparatus with high reliability and accuracy which inhibits the heat effects originating from the self heat-generation of the power transistor.
In order to attain the above-mentioned object, this invention is arranged as follows.
(1) In a gas flow rate measuring apparatus having a flow rate detecting element for measuring gas flow rate, an electronic circuit electrically connected to said flow rate detecting element for controlling said flow rate detecting element and outputting a signal corresponding to gas flow rate, a housing for protecting said electronic circuit housed within said housing, and a terminal for communicating between said electronic circuit and an external equipment of the outside of said housing, a power transistor is supported by said terminal.
(2) Preferably, in (1), said flow rate detecting element comprises a temperature sensing resistor which heats in response to the application of a current, and said power transistor amplifies and controls the current to said heating and temperature sensing resistor.
(3) Also, preferably, in said (1) or (2), the collector electrode of said power transistor is jointed onto said terminal and the base and emitter electrodes of said power transistor are connected to said electronic circuit through respective conductive members.
(4) Also, preferably, in said (1), (2) or (3), said terminal consists of a metal such as copper of which thermal conductivity is large.
(5) Also, preferably, in said (1), (2), (3) or (4), said electronic circuit is located within a conduit line forming a gas path, and said terminal has a portion located to the outside of said conduit line, whereby the heat generated from by said power transistor is conducted to the outside of said conduit line through said terminal.
(6) Also, preferably, in said (1), (2), (3), (4) or (5), the terminal supporting said power transistor extends to the inside of the path within which gas flows from the inside of said housing, and a portion of the terminal is exposed within the gas.
(7) Also, preferably, in said (1), (2), (3), (4), (5) or (6), the terminal supporting said power transistor has an inductance component.
(8) Also, preferably, in said (1), (2), (3), (4), (5), (6) or (7), said housing is made from a resin, and said terminal is structured by plated wiring to said resinous housing, using an MID technology.
(9) In a gas flow rate measuring apparatus having a flow rate detecting element for measuring gas flow rate, an electronic circuit electrically connected to said flow rate detecting element for controlling said flow rate detecting element and outputting a signal corresponding to gas flow rate, a housing for protecting said electronic circuit housed within said housing, and a terminal for communicating between said electronic circuit and an external equipment of the outside of said housing, said housing is made from a resin, said resinous housing is formed by metal plate insert mold shaping, and a power transistor is supported to said metal plate.
(10) Preferably, the terminal for communicating with said external equipment is made from a metal, and said metal plate is formed together with said metal terminal and after the insert shaping into said resinous housing it is separated from said metal terminal.
In the gas flow rate measuring apparatus, the power transistor is supported by a terminal of the outside of a housing including the circuit board.
With this arrangement, it is possible to heat-radiate the amount of heat-generation of the power transistor to the terminal, to reduce the rise of temperature of the power transistor per se, and to prevent the circuit board from being heated directly.
When said terminal is made of copper family material having high heat conductivity, it is possible to increase heat-radiation effectiveness.
Also, the terminal supporting the power transistor has such a shape that it extends from the outside of the housing to the position at which it is exposed to intake gas in the inside of the housing. Heat-radiation is carried out through the exposed terminal, thereby to improve heat-radiation effectiveness.